The present invention relates to digital printing and, more particularly, to a method and apparatus for electrostatic printing using a liquid toner.
Electrostatic printing is an effective method of image transfer. In a typical electrostatic printing process, a latent electrostatic image is first formed on an imaging surface (e.g., a photoconductor drum) by forming a uniform electrostatic charge on the imaging surface, and exposing it to a beam of light modulated by the image to be printed. The exposure procedure results in charged and discharged portions of the imaging surface, whereby charged portions form the print image and discharged portions form the background thereof. The latent electrostatic image is then developed, for example, by applying toner which adheres to the charged portions of the surface. The toner is subsequently transferred onto a print substrate, such as a sheet of paper.
One method to transfer the toner is by passing the print substrate between a roller and the imaging surface. During the toner transfer, electrostatic forces between the roller and the toner attract the toner away from the surface of the photoconductor drum onto the substrate. The toner is then subjected to a fixation process, also known as fusing, to the substrate.
However, it is impossible in practical application to transfer all of the toner to the transfer substrate, and a residual amount remains on the imaging surface and must be removed prior to a subsequent electrostatic printing operation. From an economic standpoint, it is desirable to recycle rather than discard the residual toner after such removal.
The toners used in electrostatic printing must have a number of different properties for each step in the process. For example, in order to adhere the toner to the electrical latent image in the developing step, the toner must maintain a suitable 30 amount of charge without being affected by the temperature or humidity of the surrounding environment. Also, in a fixation step in which a heated roller fixing system is used, the toner must have an anti-offset property so as not to stick to heated rollers, while having satisfactory fixability onto the substrate. Blocking resistance is also required to prevent the toner from undergoing blocking during storage in the apparatus.
Electrostatic printing may employ either dry toner or liquid toner (e.g., liquid ink). The quality of the image is related to the size of the toner particles. While it is thought that very fine particles will produce a finer image, there is a practical limitation on the size of toner particles that can be used. Dry toner particles must be of sufficient weight and size to be deposited onto the print surface without becoming airborne, which is thought to lead to machinery forming and, possibly, environmental problems. Additionally, it is difficult to recycle an electrostatic printing employing dry toner cannot be based on the use of recyclable toner, because the removal and collection of residual dry toner particles for the purpose of re-use is hampered, e.g., by the forces of dry friction.
Liquid toners have the advantage of being dispersed in a solvent, thus facilitating the use of very fine colorant particles without concern for the particles becoming airborne. In addition, the recycling of liquid toner is commonly practiced in the art of electrostatic printing because the residual liquid toner can be allowed to flow downwardly under the force of gravity. Liquid toners are obtained by mixing a certain amount of toner in a carrier liquid, which is typically selected to be a highly resistant or insulating liquid (e.g., petroleum solvent), so as to facilitate efficient toner transfer.
In addition, offset-preventing and release facilitating oil, such as silicone oil, is often used so as to increase the efficiency of toner transfer from the imaging surface.
When using liquid toners, there is a need to remove the carrier liquid from the imaging surface after the toner has been applied thereto. This prevents the carrier liquid from being transferred from the imaging surface to the print substrate. Removal of the carrier liquid is necessary for various reasons, including recycling, environmental concerns and image quality (e.g., mechanical strength). A conventional electrostatic printing apparatus therefore employs a squeegee roller or another device which removes excess liquid from the imaging surface and partially dries the liquid image prior to the toner transfer process.
The removal of the excess liquid typically results in a viscous film on the blanket, which includes more than 90% solid particles, with the balance being the carrier liquid. While the transfer process is typically performed under elevated temperature (e.g., 90° C.), during and subsequent to the transfer process most of the remaining carrier evaporates.
Due to the high viscosity of the film, the carrier evaporation sometimes results in the formation of a non-continuous and/or non-uniform film onto the substrate. The performance of the transfer process may further be adversely affected by less than optimal adhesion of the toner particles to the substrate.
The non-optimal adhesion of the toner particles and the formation of a non-continuous and/or non-uniform film onto the substrate may lead to at least partial peeling and flaking of the image from the substrate.
In order to improve the fixing properties of the toner, namely the peeling and flaking resistance, a variety of substances and/or techniques have been used hitherto. These include, for example, linear and cross-linked binding resins (e.g., polyesters, styrene-acrylic resins and the like), as well as other additives (e.g., oils, carboxylic acids) or increasing the pigment loading, which results in reduced amount of the liquid carrier and thus in a more rapid drying process. Exemplary substances and/or techniques for improving the liquid toner fixation are described for example, in U.S. Patent Application Publication No. 20040219448, and U.S. Pat. Nos. 6,656,655 and 6,140,002.
However, while the presently known substances may provide for enhanced fixability, the incorporation thereof in the toner oftentimes affects the charge characteristics and the humidity resistance of the toner, which may adversely affect the quality of the resulting image, as discussed hereinabove. Increasing the pigment loading may similarly adversely affect the image quality.
In a search for methods of improving the performance of liquid toners images, particularly with respect to the adhesion of the toner particles, the continuity and the uniformity of the formed image, the present inventors have envisioned that incorporating a UV-curable component in a liquid toner and UV-irradiating the substrate after the liquid toner is transferred thereto, would result in enhanced adhesion, continuity and/or uniformity of the toner particles to the substrate and thus images with high performance in terms of peeling and flaking resistance would be obtained.
Some UV-curable liquid toners have been reported in the art. U.S. Pat. Nos. 6,653,041, 5,395,724 and 5,212,526, for example, teach liquid toners in which at least a major portion of the liquid carrier and typically the entire liquid toner is UV-curable. Thus, according to the teachings of these patents, toner particles are suspended or dissolved in a UV-curable resin, which serves as the liquid carrier. Exemplary UV-curable liquid carriers that are taught in these patents include monomers, dimers and oligomers of acrylates and methacrylates, vinyl ethers, styrenes, indenes, alpha-olefins, butadienes, and the like. As is discussed hereinabove, such compounds, when forming the liquid carrier in electrostatic printing, may possibly affect the charge characteristics and the humidity resistance of the toner, which, in turn, may adversely affect the quality of the resulting image.
U.S. Pat. No. 5,905,012 discloses an imaging process in which a dry toner image is formed and fused on a substrate and UV-curable toner particles are thereafter deposited and cured on the image. This process therefore involves the use of two different types of toners and therefore substantially reduces the efficiency as well as the cost-efficiency of the process.
There is thus a widely recognized need for, and it would be highly advantageous to have a novel liquid toner composition for electrostatic printing and an imaging process and apparatus utilizing same, for providing images with exceptional resistance to peeling and flaking, devoid of the above limitations.